Imaging devices, such as digital still cameras (hereinafter termed simply digital cameras) and surveillance TV cameras, conventionally use image detecting elements such as CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) image detecting elements. In general, these imaging devices are required to have high image quality, including high resolution, and a compact configuration. Such performance demands that the imaging lens itself, which is mounted in each such device, be compact even when a zoom lens is used. Additionally, a so-called low-pass filter is often placed between a zoom lens and the image plane in order to reduce the high spatial frequency components of the object light for imaging. Therefore, an appropriately long back focus is needed to provide room for such a low-pass filter.
Simple zoom lenses conventionally use two lens groups. Examples of two-group zoom lenses are disclosed in Japanese Laid-Open Patent Applications H09-258103, H10-232350, H11-142734, 2000-9997, 2000-267009, 2001-21806, and 2001-100098, and Japanese Patents 3033138 and 3033149. The zoom lenses disclosed in these publications do not adequately achieve the small size and high image quality desired. For example, the zoom lenses disclosed in Japanese Laid-Open Patent Applications 2000-267009 and 2001-100098 use a relatively large number of lens elements. This tends to make the lenses too large and too complex. Furthermore, few of these publications discuss how focusing is achieved. However, image quality varies with focusing changes and the manner in which focusing is achieved, and therefore the design of the zoom lenses should consider those factors. However, that has not been properly done in the cited prior art.